There are many uncharted islands, at least one per map.
In Link and the legend of Zelda, Phantom Hourglass, you get the hammer in the isle of ruins, or the island of the dead.
it is on eddo's island
it is one of the uncharted island in the 2nd sea chart
At the Goron island dungeon
you get it in the temple of courage at modlia island
in the ice temple on frozen island
In the temple
boomarange
go to gust island
the fourth
In the temple on Goron Island
the bombuchu bag is on goron island